Evidence from patients with Alzheimer's disease suggest that the basal forebrain cholinergic system plays an important role in memory functioning. Our resuls from studies in normal monkeys show that compounds that interfere with cholinergic mechanisms, such as the cholinergic receptor blocker scopolamine, produce impairments in recognition memory. In addition, our results suggest that the effects of scopolamine are mainly anterograde, implying an action on storage rather than retrieval. In a separate study on habit formation, we administered the dopaminergic neurotoxin MPTP. This compound failed to impair learning at doses that did not disrupt motor function. Based on previous results suggesting that THC may be exerting its effects through an action on the limbic system, we administered this drug to monkeys performing spatial reversal, a task known to be sensitive to hippocampal damage. Doses of THC that impaired recognition memory did not affect performace on this task.